The present invention relates to isolation circuitry, and more particularly to novel circuitry for providing a variable-amplitude D.C. analog voltage isolated from, but responsive to, the duration of a periodic, variable pulse-width-modulated signal.
Many systems require that a relatively noise-immune signal, such as a pulse-width-modulated signal and the like, be transmitted from one location to another, with recovery of a D.C. analog signal being effected at a receiving location, for control of load apparatus. By way of example, due to the rising desire for reduced costs and increased efficiency in the area of energy management, it is desirable to realize appreciable savings in the control of the light output level of commercial fluorescent illumination systems. Such systems may control the output of fluorescent lamps at a number of locations by transmitting light output level control information from a central facility. Each remote fluorescent lamp may be energized by an associated ballast, such as is described and claimed in co-pending patent application Ser. No. 177,945, filed Aug. 14, 1980, now U.S. Pat. No. 4,357,422, assigned to the assignee of the present application, and incorporated herein by reference. The variable-luminous-output ballast/lamp combination thereof engenders cost and energy savings if: constant light output is maintained over the life of the lamp by adjustment to compensate for adverse effects, such as a cummulation of dirt and lumen depreciation; the lamp output is controlled as a function of the available natural daylight illumination; and lamp output is controlled as a function of both time and demand in a particular area. By use of interface circuitry such as described and claimed in co-pending application Ser. No. 242,782 filed Mar. 11, 1981, now U.S. Pat. No. 4,345,200, also assigned to the assignee of the present application incorporated herein by reference, the output-level-setting and on/off functions of an aforementioned ballast/lamp combination can be controlled by the magnitude of a single D.C. analog voltage signal. However, transmission of a variable amplitude analog signal over long distances suffers by pickup of random noise and other undesired signals, affecting the control signal amplitude. It is preferable to transmit control signals wherein a signal characteristic other than the signal amplitude, such as by modulation of a pulse width, conveys the desired information. Such transmission requires that circuitry be provided at each remote location for isolating the pulse-width-modulated (PWM) signal circuitry from subsequent analog signal circuitry (which may have potentially hazardous voltage and/or current levels therein) and also for converting the PWM signal to a D.C. analog level. The isolation and conversion circuitry must also have low cost, to maximize cost savings of such an energy management system.